Electrical power systems can be found in virtually all industrial areas, and they normally involve some form of power switching equipment for controllably transferring electrical power or energy to the intended load. Electrical power switching is used in a wide variety of applications such as locomotive traction, automobiles, conveyor systems, escalators and elevators, air conditioning equipment, appliances, microwave systems, medical equipment, laser drivers and radar applications.
A particular example of a commonly used power system is a power modulator, which can be regarded as a device that controls the flow of electrical power. When a power modulator is designed for generating electrical pulses it is also referred to as a pulse modulator or pulse generator. In its most common form, a power modulator delivers a train of high power electrical pulses to a specialized load. By way of example, high power electrical pulses are utilized for powering microwave amplifier tubes in driving electron accelerator systems and/or microwave generating systems for applications such as medical radiation applications and radar applications. Most of the world's high power radar sets use modulators to deliver power pulses to a microwave source, which, in turn, feeds the power, in the form of periodic bursts of microwaves, to an antenna structure. Of course, many other applications also exist. The quality requirements on the generated pulses may be high. Pulse energy, pulse width, rise time, fall time and pulse flatness are some of the quality parameters usually under consideration.
In the decades since World War II, the basic structure of power modulators has not changed significantly. A traditional power modulator consists of a power supply, which receives power from an AC power line, steps up the voltage, rectifies the power to produce direct current (DC) power, and is used to deliver energy to a reservoir, usually formed by an energetic capacitor bank. This is necessary because the input power line cannot deliver the peak power that is normally required, so the reservoir is used to deliver the peak power in small bites of energy, and is replenished or refilled by the DC power supply at a reasonably constant rate with much lower average power. Part of the energy in this reservoir is then transferred to a second smaller reservoir, usually a so-called pulse-forming network (PFN), which is normally based on several interconnected inductors and capacitors.
The PFN is rapidly charged to for example 20 kV and then momentarily connected to a pulse transformer by a high-voltage switch to deliver half the charging voltage to the pulse transformer. The high-voltage switch is typically a plasma or ionized-gas switch such as a hydrogen thyratron tube that can only be turned on but not turned off. Instead, the PFN is required to create the pulse and deliver power to the load in the form of a rectangular pulse with a relatively fast rise and fall-time as compared to the pulse width. The PFN discharges in a traveling-wave manner, with an electrical pulse wave traveling from the switched end to the “open circuited” end, reflecting from this open circuit and returning toward the switched end, extracting energy from the energy storage capacitors as it travels and feeding the energy into the pulse transformer. The pulse ends when the traveling wave has traversed the PFN structure in both directions and all the stored energy has been extracted from the network. The PFN voltage before switching is V, and the voltage applied to the primary side of the pulse transformer is V/2 or a bit less.
If a component in the PFN fails, it is necessary to re-tune the PFN for optimal pulse shape after the component is replaced. This is laborious and dangerous work, as it must be done with high voltage applied to the PFN. Besides, if a different pulse width is needed, it is necessary to replace and/or re-tune the entire PFN structure.
Having delivered the pulse, the PFN must be recharged completely to voltage V for the next pulse. To maintain a pulse-to-pulse repeatability of a few tenths of one percent, this large charging voltage “swing” must occur with great precision. Also, fully charging and fully discharging all the PFN capacitors for each pulse, several hundred to several thousand times per second, puts a heavy strain on the dielectric material in these capacitors, and this forces the capacitors to be designed with very low stress and hence a very low energy density. This makes the PFN a quite large structure.
All conventional power modulators that are based on high-voltage PFN switches, such as a thyratron or silicon-controlled rectifier switch, have a problem if a short circuit occurs at the load (as happens frequently with magnetron tubes for example). These modulators can not be turned off during the pulse, and very large fault currents can develop that sometimes damage both the modulator (particularly the switches) and the load. There is no way to interrupt the flow of current, as the high voltage PFN switch can not be turned off until the current falls to zero.
For the interested reader, general information on conventional pulse generators can be found in Vol. 5 of the M.I.T. Radiation laboratory Series on Radar: “Pulse Generators”, edited by Glasoe and LeBacqz, Wiley, N.Y. (from the late 1940's).
U.S. Pat. No. 5,905,646 relates to a novel power modulator concept using one or more switches 20 that is/are electronically controllable at both turn-on and turn-off to more or less directly connect the power source 10 to a pulse transformer 30 and/or load 40, as schematically illustrated in FIG. 1. The power source 10 is normally based on one or more energy storage capacitors that are charged by a DC power supply. The pulse width is electronically controlled by a control circuit 22 that triggers the switch to turn-on to start the pulse and to turn-off to terminate the pulse. To ensure sufficient pulse flatness, a specialized circuit can be utilized to compensate for a voltage droop during capacitor discharge. This novel type of modulator, which is sometimes referred to as the LCW modulator after the inventors Lindholm, Crewson and Woodburn, provides several advantages over the traditional PFN-based modulators:                The need for PFN networks is eliminated;        More compact constructions can be obtained;        Smaller stray losses;        Longer expected lifetime;        The pulse width can be adjusted electronically. No circuit changes or re-adjustments are needed. In the limit, the pulse width can even be changed from pulse to pulse if this should ever be needed.        The voltage delivered to the load is the same as the capacitor voltage, not half as with PFN-based modulators. This means that the full rated power of the switch can be used rather than half the power (full voltage and current).        
FIG. 2 is a schematic circuit diagram of an exemplary LCW type modulator according to the prior art. The power source 10 is basically a DC power supply that charges an energy storage capacitor. An electronically on-off controllable switch 20, such as an IGBT (Insulated-Gate Bipolar Transistor) switch, connects the capacitor to the primary side of a step-up pulse transformer 30 via a passive pulse flattening network.
Although the LCW modulator constitutes a significant advance in modulator technology, there is a price for the above advantages. The simple and more or less direct connection between the charged capacitor(s) and the load exposes the switch to possibly destructive currents and voltages if there is a load fault such as a short-circuit.
Some modern solid-state switches such as IGBT (Insulated-Gate Bipolar Transistor) switches have a built-in protection against short-circuits. However, interrupting high currents generally shortens the lifetime of IGBTs in an unpredictable way. Although some IGBTs, with a DC current rating of 1600 amps, have a “10×” short-circuit current rating, meaning that they should be able to interrupt up to 10 times the rated DC current or 16,000 amps in about ten microseconds, this is actually a “once in a lifetime” event for the switch, and is generally not meant to be repeated.
Consequently, there is a general need to protect the switches from load faults such as short-circuit faults to prevent the switch from being destroyed and/or to preserve its lifetime.